The present invention relates to lead-acid cells and batteries, and, more particularly, to a method for making positive grids using calcium-tin-silver lead-based alloys.
Over the last 20 or so years, there has been substantial interest in automotive-type, lead-acid batteries which require, once in service, little, or more desirably, no further maintenance throughout the expected life of the battery. This type of battery is usually termed a xe2x80x9clow maintenancexe2x80x9d or xe2x80x9cmaintenance-free battery.xe2x80x9d The terminology maintenance-free battery will be used herein to include low maintenance batteries as well. This type of battery was first commercially introduced in about 1972 and is currently in widespread use.
It has been well recognized over the years that lead-acid batteries are perishable products. Eventually, such batteries in service will fail through one or more of several failure modes. Among these failure modes are failure due to positive grid corrosion and excessive water loss. The thrust of maintenance-free batteries has been to provide a battery that would forestall the failure during service for a period of time considered commensurate with the expected service life of the battery, e.g., three to five years or so.
To achieve this objective, the positive grids used initially for maintenance-free batteries typically had thicknesses of about 60 to about 70 mils or so. The batteries were likewise configured to provide an excess of the electrolyte over that needed to provide the rated capacity of the battery. In that fashion, by filling the electrolyte to a level above that of the top of the battery plates, maintenance-free batteries contained, in effect, a reservoir of electrolyte available to compensate for the water loss occurring during the service life of the battery. In other words, while the use of appropriate grid alloys will reduce water loss during the service life of the battery, there will always be some water loss in service.
The principal criteria for providing satisfactory positive grids for starting, lighting, and ignition (xe2x80x9cSLIxe2x80x9d) automotive lead-acid batteries are stringent and are varied. In general, and by way of a summary, suitable alloys must be capable of being cast into satisfactory grids and must impart adequate mechanical properties to the grid. Still further, the alloys must impart satisfactory electrical performance to the battery in the intended application. Satisfactory alloys thus must impart the desired corrosion resistance, and avoid positive active material softening that will result in a loss of capacity.
More particularly, and considering each of the criteria previously summarized, suitable alloys in the first instance must be capable of being cast into grids by the desired technique, i.e., the cast grids must be low in defects as is known (e.g., relative freedom from voids, tears, microcracks and the like). Such casting techniques range from conventional gravity casting (xe2x80x9cbook moldsxe2x80x9d or the like) to continuous processes using expanded metal techniques and to a variety of processes using alloy strips from which the grids are made, e.g., by stamping or the like.
The resulting cast grids need to be strong enough to endure processing into plates and assembly into batteries in conventionally used equipment. Even further, suitable grids must maintain satisfactory mechanical properties throughout the expected service life. Any substantial loss in the desired mechanical properties during service life can adversely impact upon the battery performance as will be more fully discussed hereinafter.
Considering now the electrochemical performance required, the grid alloy for the positive plates must yield a battery having adequate corrosion resistance. Yet, the use of a continuous direct casting process, or other processes using grid alloy strips, desirable from the standpoint of economics, ostensibly can compromise corrosion resistance. Continuous processes thus orient the grains in the grids, thereby making the intergranular path shorter and more susceptible to corrosion attack and to early failures. Casting a thick strip and then cold rolling or the like to the grid thickness desired even further exacerbates the problem.
Positive grid corrosion thus can be a primary mode of failure of SLI lead-acid batteries, particularly at higher ambient temperatures. When positive grid corrosion occurs, this lowers the electrical conductivity of the battery itself. Battery failure occurs when the corrosion-induced decrease in the conductivity of the grid causes the discharge voltage to drop below a value acceptable for a particular application.
A second failure mechanism, also associated with positive grid corrosion, involves failure due to xe2x80x9cgrid growth.xe2x80x9d During the service life of a lead-acid battery, the positive grid corrodes; and the corrosion products form on the surface of the grid. In most cases, the corrosion products form at the grain boundaries and grid surface of the positive grid where the corrosion process has penetrated the interior of the xe2x80x9cwiresxe2x80x9d of the grid. These corrosion products are generally much harder than the lead alloy forming the grid and are less dense and thus occupy a larger volume. Due to the stresses created by these conditions, the grid alloy moves or grows to accommodate the bulky corrosion products. This physical displacement of the grid causes an increase in the length and/or width of the grid. The increase in size of the grid may be non-uniform. A corrosion-induced change in the dimension of the grid is generally called xe2x80x9cgrid growthxe2x80x9d (or sometimes xe2x80x9ccreepxe2x80x9d).
When grid growth occurs, the movement and expansion of the grid begins to break the electrical contact between the positive active material and the grid itself. This movement and expansion prevents the passage of electricity from some reaction sites to the grid and thereby lowers the electrical discharge capacity of the cell. As this grid growth continues, more of the positive active material becomes electrically isolated from the grid and the discharge capacity of the cell decays below that required for the particular application. The mechanical properties of the alloy thus are important to avoid undue creep during service life.
As is now appreciated, what has occurred in the last several years is the substantial increase in the under-the-hood temperature to which the battery is exposed in automobile service. Obviously, the under-the-hood temperature is particularly high in the warmer climates. One automobile manufacturer has perceived that the temperature to which an SLI battery is exposed under-the-hood in such warmer climates has risen from about 125xc2x0 F. to about 165xc2x0 F.-190xc2x0 F. in new automobiles.
The specific temperature increase which is involved is not particularly important. What is important is that such under-the-hood temperatures have in fact increased. The impact of the under-the-hood vehicle service temperature increases on the failure modes has been to substantially increase the occurrence of premature battery failures. The incidence of premature battery failures due to excessive positive grid corrosion has been significant.
A breakthrough was achieved in utilizing the positive grid alloys disclosed in U.S. Pat. No. 5,298,350 to Rao. Utilizing such positive grid alloys provided batteries that exhibited substantial improvements in service life and have effectively eliminated premature positive grid corrosion at elevated temperatures as being the primary mode of failure.
The subject Rao patent has spurred considerable interest in the type of positive grid alloys utilized, i.e., calcium-tin-silver lead-based alloys. Thus, substantial effort has been made to investigate this type of alloy through testing of various properties with varying levels of the alloying constituents.
The interest has also extended to utilizing this family of alloys in sealed lead-acid cells and batteries (often termed xe2x80x9cVRLA,xe2x80x9d viz., valve-regulated lead-acid). Sealed lead-acid cells and batteries are widely used in commerce today for various applications. In one type of application, generally termed as stationary applications, lead-acid cells and batteries are used, for example, for load leveling, emergency lighting in commercial buildings, as standby power for cable television systems, and in uninterruptible power supplies. The uninterruptible power supply may be used to back up electronic equipment, such as, for example, telecommunication and computer systems, and even as a back up energy source for entire manufacturing plants. When the principal power supply to the electronic equipment or the like has been cut off, such as during a power outage, the sealed cells (typically many electrically connected together) provide a source of reserve power to allow the telecommunication or computer system to remain operational until the principal power system can be restored. The uninterruptible power supply also will accommodate short, or intermittent, losses in power, so that the function of electronic equipment will not be impaired during a brief power outage.
In addition, there are many applications where sealed lead-acid cells and batteries are used in what are termed motive power applications. Such applications are thus electrical vehicles, fork-lift trucks, and the like, where such cells and batteries are used as the power source.
In many of these applications where sealed cells and batteries are used, the size of such cells and batteries and the necessary service life requirements necessitate that relatively thick grids be utilized in relation to the thickness of grids typically utilized for SLI applications. More particularly, grid thicknesses of 0.1 inch or more are often required.
What has also occurred in the last several years are a variety of processes which utilize alloy strips to make grids, and often in a continuous, or semi-continuous, fashion. The desirability of such continuous plate-making processes is to achieve higher grid production rates as well as to improve plate quality in comparison to the production and quality issues associated with using conventional gravity cast techniques. One process for making a directly cast alloy continuous strip from molten lead alloys is commercially available (Cominco Ltd., Toronto, Canada). U.S. Pat. No. 5,462,109 to Vincze et al. discloses a method for making a directly cast strip. This directly cast strip can then be converted by known expanded metal fabrication techniques to achieve a continuous source of an expanded lead-alloy grid mesh strip suitable for conversion into positive lead-acid battery plates. U.S. Pat. No. 5,434,025 to Rao et al. discloses batteries and positive grids made from cast strips which achieve high temperature corrosion resistance while utilizing calcium-tin-silver lead-based alloys.
Other types of grid manufacturing processes involve first casting a continuous length billet having a thickness in the range of, for example, 0.25 to 1.0 inch. Such a billet is then mechanically rolled continuously to a thickness reduction in the range of 10-15:1. The finished roll strip may then be made into grids by a variety of commercially available techniques. Such techniques have often been termed xe2x80x9cexpanded metalxe2x80x9d techniques, which techniques typically involve slitting the strip and expanding the slit strip, creating a grid mesh having diamond-shaped openings, hence the reference to the xe2x80x9cexpanded metalxe2x80x9d terminology. Alternatively, die punching or any other technique proposed and/or used to make grids from the rolled strip can be employed.
What has not been appreciated, it is believed, is the substantial adverse effect upon the microstructure of the thus-rolled strips and the concomitant effect upon the desired corrosion resistance and grid growth characteristics of grids made from such strips. More particularly, in the rolling process whereby the alloy strip is created from the cast billet, the stability of the microstructure may be lowered, both non-uniform and higher rates of matrix recrystallization can result. Such results can increase the susceptibility to intergranular corrosion. These higher rates of matrix recrystallization in such rolled alloys may well be due to the excess strain energy absorbed during rolling. The recrystallization temperature of the alloy may thereby be lowered due to the excess strain energy and the magnitude of lattice defects present in the matrix, in turn, due to the heavy structure deformation at the lower recrystallization temperature.
The significance, at least in part, is that the precipitation in the matrix in Pbxe2x80x94Caxe2x80x94Sn and Pbxe2x80x94Caxe2x80x94Snxe2x80x94Ag alloys will be non-uniform; and recrystallization may well result in localized movement of large angle grain boundaries. Recrystallization results in non-uniform grain growth in the lead-rich matrix. Excessive grain boundary movements also result in pulling adjacent precipitate particles together which could coalesce to form agglomerates. This will tend to increase the precipitate size, also increasing the interparticle spacing, both of which will reduce the effectiveness of the precipitates in matrix strengthening, thereby contributing to loss of ductility and toughness. Precipitate coarsening could also lead to grain boundary precipitation and accordingly make the alloy more susceptible to catastrophic intergranular corrosion in battery life. Such matrix recrystallization may also result in reduction in the creep rate of these alloys which will, in turn, exhibit higher grid growth rates in battery service so as to limit the useful service life.
In view of the production and quality improvement capable of being achieved by making grids from cast strips, there exists a clear need for methods capable of utilizing the substantial benefits that can be achieved using the calcium-tin-silver lead-based alloys, while not unduly limiting the potential advantageous properties.
Accordingly, it is an object of the present invention to provide a method for making positive grids and plates for a lead-acid battery utilizing a rolled or wrought strip.
It is an additional object of the invention to provide lead-acid cells and batteries utilizing positive grids made with such a method.
Another object of this invention is to provide such method in which the wrought strip produced, made utilizing calcium-tin-silver lead-based alloys, is characterized by superior microstructure stability, stable and uniformly dispersed (PbAgSn)3Ca-type precipitates, lower rates of matrix strain hardening, strain energy and residual stresses, equiaxed and honeycomb grain structure, and a matrix relatively resistant to recrystallization and corrosion.
Other objects and advantages of the present invention can be seen from the following description of the invention.
In general, the method of the present invention involves carefully controlling the rolling of the billet so as to provide positive grids made from calcium-tin-silver lead-based alloys which have highly beneficial properties. Indeed, such positive grids are considered to be ideally suited for sealed lead-acid cells and batteries that are intended for relatively long term service lives. On the other hand, if desired for use in SLI lead-acid battery applications and the like, the use of grids made using the methods of the present invention should possess such inherently high corrosion resistance that the grid thickness and weight can be reduced, if desired, by anywhere from about 5% to 10% or so. Reductions of this level provide a potential economic reward that is considerable.
As will be discussed more particularly hereinafter, the method of the present invention involves casting the billet, then rolling at a controlled temperature which is above the solvus, and is somewhat less than the peritectic, temperature for the defined calcium concentration in the alloy, quenching the rolled strip to preserve the supersaturated, lead-rich solid solution, and then maintaining the rolled strip at selected temperatures until ready for conversion into grids.
According to a more preferred embodiment of the present invention, it has been found that subjecting the rolled strip to a controlled artificial aging sequence can further enhance the corrosion resistance of grids made using the present invention. Thus, as will be discussed hereinafter, such a controlled artificial aging sequence can be utilized to reduce intergranular corrosion.